24R,25-Dihydroxyvitamin D3, lysophosphatidic acid, and p53: A signaling axis in the inhibition of phosphate-induced chondrocyte apoptosis

Maintenance of the pool of chondrocytes in the resting zone of the growth plate in the presence of the physiological apoptogen inorganic phosphate (Pi) is crucial for skeletal development. Costochondral resting zone chondrocytes are regulated by the vitamin D metabolite 24R,25-dihydroxyvitamin D3 [24R,25(OH)2D3], with increased production of sulfated glycosaminoglycan-rich extracellular matrix, and reduced matrix metalloproteinase activity. The effects of 24R,25(OH)2D3 are mediated by activation of phospholipase D (PLD), resulting in increased production of lysophosphatidic acid (LPA) and LPA-mediated proliferation, maturation, inhibition of Pi-induced apoptosis, and reduction of p53. However, the exact mechanism by which 24R,25(OH)2D3 and LPA exert their effects is not fully understood. It was found that both 24R,25(OH)2D3 and LPA attenuate Pi-induced caspase-3 activity. The actions of 24R,25(OH)2D3 and LPA were dependent upon Gαi, LPA receptor(s) 1 and/or 3, PLD, phospholipase C (PLC), and intracellular calcium, phosphoinositide 3-kinase (PI3K) signaling, and nuclear export. 24R,25(OH)2D3 decreased both p53 abundance and p53-medaited transcription and inhibited Pi-induced cytochrome c translocation. Moreover, LPA induced increased mdm2 phosphorylation, a negative regulator of p53. Taken together, these data show that 24R,25(OH)2D3 inhibits Pi-induced apoptosis through Ca2+, PLD, and PLC signaling and through LPA-LPA1/3-Gαi-PI3K-mdm2-mediated p53 degradation, resulting in decreased cytochrome c translocation and caspase-3 activity.